The present invention relates to a thermal head drive circuit for use in a thermal recording apparatus such as facimile equipment or a printer. More particularly, the invention relates to apparatus for controlling the electrical energy provided to each heating element in the thermal head in order to provide a plurality of gray shades in a picture reproduced from an electrical signal upon a thermally sensitive paper.
Thermal recording apparatus, in which recording is thermally produced by using a thermal-sensitive recording paper or other thermal-sensitive medium, is widely used in facimile equipment. In such a thermal recording apparatus, a thermal head is used for printing in which there is a linear array of individually actuatable heater elements. Thermal energy to each of the heater elements is individually controlled to determine the density of the dot-like region at the point where the heating element and the recording paper are in contact. Shades of gray between white and black inclusive, typically as many as 16 shades of gray, may be obtained by controlling the time duration of a voltage applied to each of the heating elements. In the prior art, all elements which were desired to print the same shade of gray were energized simultaneously for the required pulse duration to produce the desired shade of gray. After completion of printing of that gray shade, the pulse duration of the applied energy is reduced and applied to all heating elements under which the adjacent shade of gray was to be printed. This process of changing the pulse length of the applied energy was continued through 16 cycles thereby providing the range of black to white in 16 shades of gray. This process of printing the 16 shades of gray in a sequential manner results in a printing technique which is slower than is desirable. Consequently, a need exists for improvements in thermal head drive circuits of the type in which thermal energy is supplied to respective heater elements to obtain faster printing.